This invention relates to a purlin bracing system for a metal building roof.
Many modern metal buildings have roof panels which are supported by purlins running parallel to the roof ridge across structural beams typically defining bays. FIG. 1 is a diagram of a typical arrangement, looking along the ridge plane “R”. The purlins “P” and the eave struts “E” have high bending stiffness in the vertical direction, less stiffness horizontally. In most instances, the purlins have to be braced horizontally at intervals. Determination of the proper bracing interval is a matter of ordinary skill not forming part of this invention.
Purlins, like other long, slender structural members loaded in bending on their “stiff” axis, want to deflect laterally and twist toward a less stiff axis. Bracing must be applied at proper intervals to prevent this mode of failure. The bracing interval can be calculated from a number of parameters, including the moment of inertia on the stiff axis, the moment on the less stiff axis, the modulus of elasticity, the distribution and magnitude of the design load, and the nature of the constraints at each end of the structural member. In practice, builders follow bracing tables which are determined mathematically or empirically.
Various purlin brace designs have been proposed and used. Some are bolted in position between the purlins; others have tabs which are inserted through slots in the purlin and then are bent over to retain the brace. A good example is Parsons' U.S. Pat. No. 3,092,221.
One method which has been long used to brace purlins is to install pairs of structural angle members between the purlins. FIG. 2 shows a conventional (“prior art”) purlin brace arrangement in which a pair of structural angle members, having tabs at their ends, were installed between purlins by inserting their tabs through pre-formed slots in the purlins' webs. The tabs were then bent down to lock the parts together.
The prior designs required the use of tools of some sort. It would be an improvement to have purlin braces which could be installed quickly without tools and without specialized fasteners, and yet would remain securely in position between the purlins once they were installed.